The present invention relates to frequency-hopping filters for communications signals. More specifically, but without limitation thereto, the present invention relates to a filter to prevent strong interfering signals in nearby frequencies from overloading the amplifier and limiter stages in the front end of communications receivers.
Frequency-hopping, frequency-scanning wideband and ultrawideband communications receivers cannot employ simple narrowband preselector filters to protect amplifiers and limiters in receiver front ends from strong interference outside the communications signal bandwidth. Close proximity to multiple transmitters reduces the effective communications range of such receivers to almost zero. This range reduction has been shown to be due to intermodulation products in the front end of the receiver. Diodes near the receiver""s antenna port used for power limiting or circuit switching act as mixers. The resulting intermodulation products affect virtually every communications channel in the receiver range.
Frequency-hopping filters have been under development for VHF and UHF frequency bands, but these are expensive and require switching in tandem with the receiver frequency.
A continuing need exists for a front end filter for a frequency-hopping receiver that is not dependent on knowledge of the frequency excursions of the transmitted signal.
The comb limiter combiner of the present invention is directed to overcoming the problems described above, and may provide further related advantages. No embodiment of the present invention described herein shall preclude other embodiments or advantages that may exist or become obvious to those skilled in the art.
A comb limiter combiner of the present invention comprises an input signal coupler for coupling to a receiving antenna and distributing the antenna signal to a bank of input bandpass filters. The input bandpass filters have contiguous passbands that comprise the total receiver bandwidth. Each input bandpass filter is connected to a limiter having a threshold substantially equal to the limiting threshold of the receiver. Each limiter is connected to an output bandpass filter similar to the corresponding input bandpass filter to remove out-of-band intermodulation products generated by the limiter. The bank of output bandpass filters is connected to an output signal coupler for coupling to the front end of the receiver.
An advantage of the comb limiter combiner is that intermodulation products are restricted to the passband of a single bandpass filter.
Another advantage is that a comb limiter combiner design requires no knowledge of the frequency excursions of the transmitted signal.
Yet another advantage is that a comb limiter combiner design requires no switching or control circuitry.
The features and advantages summarized above in addition to other aspects of the present invention will become more apparent from the description, presented in conjunction with the following drawings.